1. Field of the Invention
The present invention relates to power amplifying apparatuses. In particular, the present invention relates to a power amplifying apparatus suitable for power amplification of OFDM (orthogonal frequency division multiplexing) modulated signals for a mobile communication device and a mobile communication terminal which uses the power amplifying apparatus.
2. Description of the Related Art
OFDM (orthogonal frequency division multiplexing) techniques have been developed for increasing transmission rates of communication signals used for mobile communication devices and improving frequency utilization efficiency. OFDM realizes a very high transmission rate with a very narrow frequency bandwidth, making this technique attractive especially in an environment with limited frequency resources. However, a peak factor (a ratio of a peak power to an average power) in OFDM is very large, such as 10 dB or larger, compared with that in other techniques such as CDMA (code division multiple access) in which the peak factor is approximately 3 dB. This large peak factor increases the amount of load imposed on power amplifiers for mobile communication devices or the like.
Power amplifiers for use in OFDM systems which are composed of single transistors are known. Such power amplifiers can increase the output level by no more than several dB even when distortion compensation is performed, and thus may not be effective for a signal having a peak factor as high as 10 dB. To address this shortcoming, a power amplifier in which separate amplifying elements each operating on the basis of an output power level and outputs of the amplifying elements are combined has been proposed. Examples of such a composite power amplifier include LINC (linear amplification with nonlinear component) amplifiers and Doherty amplifiers. An amplifier configuration based on Doherty amplifiers may be suitable to deal with the above shortcoming, taking into account the characteristics of OFDM systems described above (see Japanese Unexamined Patent Application Publication No. H7-022852).